In the ceaseless quest to identify players with the most raw pop, isolated power, or ISO, is a frequently cited metric, as you’ve probably seen. It’s simply a player’s slugging percentage minus his batting average. The idea is that, by removing single base hits, and measuring just a player’s extra bases per at bat, we can approach a pure measure of a power. I’ve referred to it quite a bit in the past. Like OPS, it’s simple, easy to explain, and easy to calculate in a pinch.

Recently I came across this article by Lewie Pollis from Beyond the Boxscore, wherein he resurrects a metric that had been floating around stat-minded circles in the past — power factor. Power factor is isolated power with one basic change: the denominator is a player’s hits instead of at bats. It can be expressed as isolated power divided by batting average, but what it boils down to is extra bases earned per hit. When you phrase it that way, I think the advantage is obvious, but Pollis provided a helpful mental exercise to illustrate how it more effectively approaches a player’s power:

Let’s take two hypothetical players: Tony Stark and Bruce Wayne. Stark hits .350 with a .550 SLG. Wayne hits .200 with a .400 SLG. Both weigh in at a .200 ISO, suggesting their raw power is roughly equal.

But that’s not right. If Wayne somehow managed to bring his average up 150 points, would all the extra hits be singles? Stark is clearly a better hitter, and they both produced the same amount of extra bases. But if the two players had similar contact skills, Wayne would undoubtedly be a bigger slugger.

Simply put, a player’s isolated power is still influenced by his batted ball fortune and contact skill, which are not things we want to factor in to our evaluation of power. Measuring extra bases per hit, instead of per at bat, negates these influences. Take the 2011 performances of Ryan Howard and Shane Victorino as an example. Their ISOs are .236 and .234 respectively, suggesting that their power hitting is roughly equivalent this year. But Howard gets .952 extra bases per hit (his power factor), while Victorino only manages .767. Clearly, if their batted ball luck and contact skills were equal, and Howard were getting as many hits as Victorino, his power as measured by ISO would be superior. That’s what power factor attempts to get at.

Put that aside for a second, and let’s look at another attempt to tweak isolated power for the better. At Fangraphs, Steve Slowinski recently developed a metric he calls Weighted Extra Bases. Instead of using the extra base count of each extra base hit (1 for doubles, 2 for triples, 3 for home runs), Weighted Extra Bases uses their linear weights values (the same ones that go into the calculation of wOBA) in an attempt to represent each hit type’s true value in a power metric. As Slowinski writes, the formula goes:

When you actually weigh extra base hits in proportion to their value, it turns out that ISO is undervaluing doubles, while overvaluing both triples and home runs. This is why a player like Matt Holliday (33 doubles, 19 home runs) can have the 9th highest wXB/AB in the majors, yet only the 20th highest ISO and 17th highest SLG.

As you can see, doubles have more value than we usually consider, and more than ISO credits them for. The second extra base gained on a triple, on the other hand, is not quite as valuable as ISO calculates it to be. Home runs are similarly overvalued. Using Slowinski’s twist, we can measure a player’s power with a more accurate accounting of the contribution to run scoring that it provides.

So we have two separate variations on isolated power, each resolving a separate issue: Pollis’ use of Power Factor eliminates the influence of contact skills and batted ball luck in measuring a player’s power, and Slowinski’s Weighted Extra Bases paints a clearer picture of the value that each extra base hit type provides, and therefore a clearer picture of the player’s power contribution. Now why not combine them, harvesting the advantages of each? Specifically, let’s take Weighted Extra Bases as our numerator, and hits (rather than at bats) as our denominator. This would allow us to see the true power value that a hitter provides with each hit he collects. Now, Holliday’s doubles are not undervalued, and Victorino’s contact ability and BABIP are not unduly represented. Two birds with one Randy Johnson fastball. The new formula is:

The names aren’t surprising, but this looks quite a bit different from the best 15 ISO seasons using the same criteria. McGwire appears with the same frequency, but players held up by high contact skills, such as Babe Ruth, Lou Gehrig, Jimmie Foxx, are bumped down a bit. And players who did a lot with the ball when they did hit it, like Jose Bautista and Carlos Pena, are given their proper due. Players who appear on the Power Factor leaderboard for the same criteria but are boosted by overvalued hit types, like Adam Dunn, Harmon Killebrew, and Sammy Sosa, are also bumped down in the wXB/H metric.

Now for the career leaders. Who, according to wXB/H, provided the most value with their power every time they logged a hit? Unsurprisingly, it turns out to be McGwire, and by a wide margin at that (minimum: 3000 At Bats):

I know what you’re thinking now: Gorman Thomas? Steve Balboni? The former spent 13 years in the majors, from 1973-1986, hitting decently above average (114 OPS+), which was not bad for a guy who was a full time centerfielder for most of his career. But he’s not exactly the stuff of legends, and he certainly doesn’t come to mind if you were asked to rattle off some of the great power hitters in history. His slugging percentage only cracked .500 twice. This is mostly because he didn’t have very good contact skills — his career batting average was just .225. But when he did get a hit, it turned out to be an extra-base hit 47% of the time. The league average for that figure is 29%, and only a handful of other players have sustained a better rate than him for the amount of time he did. The same can be said for Steve Balboni. He was a league average hitter (101 OPS+) for 11 seasons, batting only .229 for his career, but 45% of his hits went for extra bases. Our all-time leader McGwire, by the way, turned 52% of his hits into extra base knocks.

How about the Phillies? Here are the top 20 wXB/H seasons in franchise history:

Nothing groundbreaking. Mike Schmidt appears seven times. His top ranking season, 1979, was only his 6th best by OPS+, but he posted a .564 slugging percentage while hitting just .253. He homered in 6.7% of his plate appearances (league average that year was 2.1%), and his hits went for extra bases 54% of the time. Almost all of Ryan Howard’s career is represented. It’s interesting, though, that Howard’s best offensive season, 2006, ranks the lowest of his other seasons on this list. Both his SLG and ISO that year were, by far, the highest of his career thus far. But again, we’re seeing the effect of eliminating contact factors; Howard’s batting average and BABIP were much higher in 2006 than in subsequent years, and when he connected with the ball from 2007-2009, it went for extra bases more frequently.

There are perhaps some surprising names (hello Cy Williams), and you could quibble about the order, but the top dogs are as you might expect. For all the intra-fanbase battles about Howard’s value, his power is undeniable, and will be his legacy as a Phillie. There is a wide gap separating him, Schmidt, and Burrell from the rest. By wXB/H, at least, they represent the top tier of raw pop in Phillies history.

It’s an interesting metric to play around with, and I think it gets at some aspects of a player’s hitting that might not necessarily be represented by slugging percentage or isolated power. I certainly would not go bandying it about in overall player evaluations, but raw power is one of those fundamental things that draw us to baseball, and that make it fun to watch, and it’s not as easy to quantify as it might seem on its face. As a postscript, here are the values for the 2011 qualified Phillies: